The Case of the Watery Grave

Excerpted from: Mysteries of the Distilled Water Ice Bath, Superior Science, Stories of Lake Superior Research

Martin Auer

Professor Martin Auer, a veteran lake explorer from the Department of Civil and Environmental Engineering at Michigan Technological University, thrives on questions. He's the type who idolizes the late marine ecologist Jacques Cousteau for his insatiable curiosity and who will disassemble a child's toy to satisfy his own. He's the type to describe Lake Superior as an "ice bath of distilled water," then proceed to illustrate how even this relatively cold, sterile lake is awash with mysteries.

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One of the first mysteries Auer likes to talk about is set in a graveyard – of sorts. The plot involves a blanket of chlorophyll-packed algae that appears each July nearly 30 meters below the surface of Lake Superior. The mysterious green blanket even has an enigmatic name: the deep chlorophyll maximum.

Phytoplankton distribution in July illustrates the "deep chlorophyll maximum" in Lake Superior

"The deep chlorophyll maximum is a wretched place for algae to live," said Auer. "There is no way they (the algae) are down there because they like dim light or cold temperatures. Yet there are a lot of algae hanging on the transition zone between warmer surface water and the very cold water below. The question is whether the algae congregate there because of some attractive element or whether it's a settling place for dying algae."

After the lake mixes each spring, algae are scattered sparingly, at about half a microgram per liter throughout the water column. As April progresses towards June, rainwater entering from land and favorable light conditions in shallow waters encourage algae to reproduce near the shore. But then, by a typical July, Lake Superior becomes thermally stratified with a defined layer of warmer water resting atop colder water. Like frosting between layers of a cake, a sharp transition zone and the deep chlorophyll maximum mark the interface between the layers of temperature. Algae in the deep chlorophyll maximum occur in three to five times their April concentration.

The bulk of algae production occurs in the upper 20 meters of water, where the combination of heat, light, and nutrients (phosphorous) allow these single-celled plants to thrive. In the deep chlorophyll maximum, the temperature is about 5deg; C, ten degrees colder than optimal for alga growth, and the sun's light is dimmed to about 20 percent of ideal. Despite abundant and available phosphorus, Auer thinks the low light and cold temperature make the deep chlorophyll maximum more akin to an elephant graveyard, where algae go to die, than a fertile plain.

"An abundance of phosphorous is inconsequential to algae in the deep chlorophyll maximum," said Auer. "It's like having money in your pocket but all the stores are closed."

Auer continues with his favorite explanation about why the algae came to rest in the deep chlorophyll maximum.

"It's not that they (the algae) like to live there, it's a physical thing for them. They are just sort of stuck there. After mixing, the algae sink until they hit denser water where they eventually die."

Unlike the algae (which are sinking to their death), Auer reports that bacteria "are growing like crazy" in the deep chlorophyll maximum and that their carbon requirements offer clues about what the algae are doing 30 meters under the sea. Bacteria need carbon. Research indicates that it would be impossible for the available photosynthesizing phytoplankton in the deep chlorophyll maximum to supply the thriving bacteria populations with the amount of carbon necessary for the level of reproductive activity they exhibit. Evidently, the carbon sources utilized by bacteria in the deep chlorophyll maximum are more characteristic of bottom waters than surface waters.

"It's the dying phytoplankton feeding the bacteria, not the carbon excreted by reproducing algae," testified Auer, thus closing the Case of the Watery Grave but not the book on Lake Superior's mysteries.

To read about other mysteries that Auer and his colleagues are pursuing, and to explore Lake Superior from the perspective of eight notable researchers, request a copy of Minnesota Sea Grant's newest book, Superior Science, Stories of Lake Superior Research. (See item S 17, education category, products order form). For a limited time, we're offering this publication for the cost of shipping, $2 each, since a grant from the Lake Superior Coastal Program paid for this first printing.